1. Field of the Invention
The present invention relates to a method for manufacturing fibrous rods, such as for cigarette filters, which have a well-stabilized shape, and an apparatus for practicing the method, in which acetate fibrous tows impregnated with carboxylic acid ester of polyol are formed in predetermined shapes or configurations and a processed using microwave energy in microwave absorbing material.
2. Description of the Prior Art
There have been heretofore proposed various methods for manufacturing fibrous rods used in cigarette or other filters. For example, a method for forming fibrous rods of plasticizer impregnated acetate fiber with a filter plug machine is disclosed in U.S. Pat. No. 3,413,689. As described in that patent, the plasticizer impregnated tow is formed into a fibrous rod in a predetermined shape and the periphery of the tow is wrapped with a wrapping paper and a continuous belt to form a filter rod. The stability of the configuration of the rod-shaped fiber wrapped with the paper is determined by the applied plasticizer. The time period required for configuration stabilization due to setting of the plasticizer is typically several hours or more. Therefore, if the wrapping paper is removed from around the rod-shaped fibers before the shape has stabilized, the rod-shaped fibers will expand radially as shown in FIG. 3. Due to this fact, it is almost impossible to produce wrapperless fibrous filter rods using this method.
A wrapperless fibrous filter rod is superior in its ability to remove harmful components in tobacco smoke such as tar although the reason for this improved ability is not known.
A method for producing wrapperless fibrous rods has been proposed in U.S. Pat. Nos. 3,313,306, 3,111,702 and 3,095,343. As described in those patents, a tow impregnated with a plasticizer is circulated and transported by a steam permeable continuous belt and is processed using a pressurized steam. Pressurized steam is used for activating the plasticizer applied to the tow and for promoting adhesion between the fiber filaments of the rod-shaped fibers to thereby stabilize the shaped articles. Using this method, it is possible to produce wrapperless fibrous rods with a high production efficiency. However, since a large amount of water remains in the filter rods, a large amount of energy is needed to remove the moisture. Also pressurized steam must be used which is difficult to handle.
Yet further, a method for manufacturing wrapperless fibrous rods using microwave energy has been proposed in U.S. Pat. No. 4,003,774 and U.K. Patent Application No. 2,030,400. In the fibrous rod manufacturing method proposed in U.S. Pat. No. 4,003,774, an acetate fiber tow to which no plasticizer has been applied is formed in a predetermined shape and is processed using microwave energy. Microwave energy is absorbed by the acetate fiber to thereby plasticize the fiber with heat and to thereby enhance the adhesion between neighboring filaments. It is possible to produce wrapperless fibers using this method. However, since acetate fiber does not have a distinct melting point and the range of thermal plasticization is very narrow above which melting immediately occurs, it is difficult to control the process used for manufacturing fibrous rods according to this method. If impregnated acetate fiber of carboxylic acid ester of polyol is used with this method, the acetate fiber, upon absorbing microwave energy, is nearly instantaneously heated to a temperature of 200.degree. C. or more. In this case, the plasticization of carboxylic acid ester of polyol is quite remarkable. For this reason, melting of the acetate fibers is much more prevalent than adhesion between the acetate fibers. It is, therefore, impossible to effectively produce filter rods having good configuration stabilization and filteration characteristics. Moreover, the plasticizer often evaporates out of the acetate tow upon the application of microwave energy. The evaporated plasticizer causes various difficulties. Therefore, using this method, it is very difficult to produce fibrous rods containing carboxylic acid ester of polyol.
The above noted defects inherent in the method proposed in U.S. Pat. No. 4,003,774 may be overcome using a method disclosed closed in U.K. Patent Application No. 2,030,440. The U.K. Application describes a method wherein a material which is soluble in the acetate fiber and which has a higher microwave absorption ratio than that of the acetate fiber and is essentially fully recoverable is used. That is, a second material is used which is not a solvent and not a plasticizer with respect to the acetate fiber. The second material, which is added to absorb microwave energy, is heated to a temperature of 190.degree. to 280.degree. C. to thus be converted from the liquid state to the vapor state. An acetate fiber tow impregnated with the second material is processed using microwave energy wherein the second material absorbs microwave energy and thereby generates heat with which the acetate fiber is melted and caused to adhere. The vaporized second material is collected and expelled. Thus, with this method, the defects inherent in the method of U.S. Pat. No. 4,030,440 are overcome. However, it is impossible to use in the filter material carboxylic acid ester of polyol. Namely, since the carboxylic acid ester of polyol is a strong plasticizer for the acetate fiber and has low microwave absorption characteristics, if acetate fiber is impregnated with carboxylic acid ester of polyol is processed using this method and formed into a fibrous rod, the various disadvantages previously described with respect to U.S. Pat. No. 4,030,440 are present and furthermore carboxylic acid ester of polyol is mixed into the collected second material. If the second material is reused, various problems arise.